Senna might be the most balanced on launch champ ever made.

Because she's a late game champ who has 40 base damage with 0.4 bonus AD ratio on her Q, a slow and easily dodged W. Even when she's fed, she has less DPS than an ADC that isn't as fed because of her 0.2 base attack speed. Even onhit effects require skill to use with her Q, because you can't just point and click somebody to proc it since it only affects the furthest target i.e. if you Q somebody in a minion wave, it'll only affect minions standing behind them.

The only problem is her ult though, which needs more telegraphy and a slightly longer travel time. Other than that, she's fine, because you can easily counter her with any early game champ such as Vi or Wukong in any lane she's in. Yeah, that's right... I played Vi as a support against Senna and won. Because who would've thought that early game champs beat late game champs in lane?